1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a strained-silicon-on insulator MOSFET with a single or a double gate, and to a method of forming a strained-silicon-on-insulator structure.
2. Description of the Related Art
A MOSFET fabricated in 001-oriented silicon under biaxial tensile strain exhibits higher carrier mobilities than a conventional MOSFET (e.g., see K. Rim, J. L. Hoyt, J. F. Gibbons, “Fabrication and Analysis of Deep Submicron Strained-Si N-MOSFET's”, EEE Trans. Electron Devices, 47(7), p. 1406, (2000)). The higher carrier mobility leads to a higher current drive and thus a faster/shorter switching time is obtained.
The “strained” silicon film is typically formed by growing an epitaxial silicon layer on top of a strain-relaxed, graded SiGe layer structure (e.g., see P. M. Mooney, Materials Science and Engineering Reports R17, p. 105 (1996) and references therein).
As known, Ge has a lattice constant which is approximately 4% larger than the lattice constant of Si, and the lattice constant of the alloy, Si1-xGex, increases approximately linearly with increasing Ge mole fraction, x, of the alloy. Since these semiconductors have cubic symmetry, the in-plane and out-of-plane lattice constants are equal in unstrained crystalline films or bulk crystals.
Herein, strained (or fully strained) means that the in-plane lattice constant of the SiGe layer, which is larger than that of the Si substrate, is compressed so that it matches that of the Si substrate (see FIG. 31A), resulting in a corresponding expansion of the out-of-plane lattice parameter such that the in-plane and out-of-plane lattice parameters of the SiGe layer are no longer equal. A SiGe layer is partially strained or partially relaxed when the in-plane lattice parameter is larger than that of Si, but still smaller than the out-of-plane SiGe lattice parameter. The SiGe is fully relaxed or unstrained when the in-plane and out-of-plane lattice parameters are equal. For Si under biaxial tensile strain (e.g., when it is grown epitaxially on a partially or fully relaxed SiGe layer), the in-plane lattice parameter is larger than the out-of-plane lattice parameter.